For combustion engines to be allowed to be sold they have to fulfil legal requirements regarding exhaust emissions. Meeting those requirements involves increasingly precise control of the engine's combustion. An important factor which affects the combustion process and hence the formation of harmful emissions and the efficiency of combustion is the air movement which occurs in the combustion chamber when the fuel is sprayed in and the combustion commences. This air movement is crucially affected by the movement imparted to the inlet air entering the cylinder as a result of the configuration of the inlet duct/ducts.
In diesel engines with double inlet valves and with relatively long inlet ducts of the type referred to in SE A 9203900-7 the movement imparted to the inlet air is at a relatively small angle relative to the piston plane and is at the same time substantially tangential relative to an imaginary circle which is smaller than and concentric with the cylinder. The ducts consequently impart to the inlet air a certain rotation which can be measured by means of laboratory facilities and be used for testing duct configurations with optimum characteristics for a chosen compromise.
The configuration of such ducts of the so-called tangential type and, more particularly, their mouth leading into the combustion chamber has been found to be of great significance for achieving the desired rotation of the inlet air. A problem which arises in the casting of large series of cylinder heads is maintaining sufficiently close tolerances on the position of the ducts. A risk which arises is that subsequent milling for the incorporation of a valve seat ring may not result in the latter being totally concentric with the duct. Irregularities affecting, inter alia, the shape of edges extending into the duct may therefore occur, with consequent adverse effects on the movement of inlet air into the cylinder, and may hence cause total disruption of the desired monitoring and control of the combustion process.
European patent specification EP-A-275841 shows a valve seat in an intake duct of an internal combustion engine. A milled recess in the inlet duct houses the valve seat equipped with bilateral chamfer. The purpose is to make the machining process for obtaining a valve seat in an inlet duct less expensive.
Swedish patent specification SE A 7501689-9 refers to the configuration of the mouth of an inlet duct leading into the combustion chamber in a diesel engine without stating how the movement of the inlet air is thereby affected. The inlet duct therein referred to is of the so-called spiral type which imparts to the inlet air a pronounced spiral-like movement which has at the same time a relatively powerful vertical component. This type of inlet air movement results in combustion which is relatively unfavourable from the emission point of view and is usually avoided in modern diesel engines.